The invention relates to the use of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) devices that are operated as switches. Such devices make excellent power switches for use in integrated circuit (IC) applications. Typically, a power MOSFET is created by fabricating a large number of small transistors into a silicon substrate and then connecting them in parallel by means of the aluminum IC metallization. By connecting several thousand such individual transistors in parallel the resulting power MOSFET can switch many amperes of current. The on resistance is a small fraction of an ohm. Such power MOSFET's can be self-isolated on an IC chip and combined with other elements using conventional bipolar junction transistor (BJT) processing and complementary metal oxide semiconductor (CMOS) fabrication to provide the circuits that operate and control the power MOSFET's in the development of so-called "smart switches".
In a copending patent application, titled CURRENT SENSING OF A DC OR A STEPPER MOTOR, Ser. No. 314,334, filed Feb. 23, 1989, now U.S. Pat. No. 5,032,745 issued July 16, 1991, a circuit is described for sensing the current flowing in a power DMOST (Diffused Metal Oxide Semiconductor Transistor), which is a form of MOSFET. The DMOST disclosed is constructed with a separate current sense emitter which provides a sense current that represents a fraction of the power DMOST current. This application is assigned to the assignee of the present invention and its teaching is incorporated herein by reference.
The aforementioned patent application also teaches what is known as an "H switch" configuration. Here, four power DMOST's are coupled together so that each of the four vertical H legs are composed of power switches and the horizontal bar represents the load being switched. Diagonally opposed DMOST's are turned off and on together by the control circuitry so that current can be passed through the load in either direction. The load current can be controlled by means of the duty cycle of the pulses used to switch the DMOST's.
The conventional DMOST incorporates a shunt diode that can act as a current snubber which makes the device conductive in both directions. When the DMOST is turned on it will conduct current between its drain and source elements and has a relatively low voltage drop. In the reverse polarity the shunt diode will conduct and, thus, produce the well-known V.sub.BE voltage drop. This means that the DMOST will conduct bilaterally and produce small voltage drops in both directions.
In the prior art smart switches the current sensing provides an output that represents the DMOST conduction. Where an H switch is involved the total current flow is sensed. It would be desirable to be able to sense the DMOST on current independently from the total current in order to better control the digital switching of the devices.